1. Technical Field
The present invention relates to an ink set having a wide color reproduction range (gamut), in particular, an ink set having a wide color reproduction range of a blue region in a dark portion and being excellent in color reproduction in other color regions. Furthermore, the invention relates to a recording method using the ink set and a recording apparatus therefor.
2. Related Art
As a color model, a CIE/L*a*b* color system is known. According to the CIE/L*a*b* color system, any color (spectrum) in a visible light region that can be observed by human beings is able to be expressed by chromaticity coordinates.
In three parameters (L*, a*, and b*) of the CIE/L*a*b* color system, L* denotes luminance (brightness) of a color, and a* and b* denote chromaticity representing hue and saturation, respectively. Here, L*=0 means black, and L*=100 means white; a* means the position of a color between red and green, wherein a negative value represents green, and a positive value represents red; and b* means the position of a color between yellow and blue, wherein a negative value represents blue, and a positive value represents yellow. Since the L*a*b* color model is three-dimensional, a color space is expressed by a three-dimensional space, wherein the luminance is shown on the vertical axis.
However, the L*a*b* color model is only an index for conceptually explaining a color space, and the color reproduction range thereof has limitation. That is, for example, in the case that an image is formed on a recording medium using a pigment-based ink or ink set, there is a color that cannot be reproduced by the colors of the L*a*b* color model.
At the same time, ink compositions containing various pigments having high chromogenicity have been proposed. JP-A-2006-282810 discloses that an ink having high chromogenicity and being excellent in ink reliability such as ejection stability, storage stability, and clogging can be obtained by an ink composition containing a quinacridone solid solution pigment, a polyether-modified polydimethylsiloxane compound, and water. JP-A-2007-186641 discloses that an ink excellent in ink reliability such as printing quality, ejection stability, storage stability, and clogging can be realized by an ink composition containing a water-insoluble vinyl polymer, water, and a surface-treated pigment (yellow, magenta, or cyan) that is composed of a solid solution pigment formed of specific two or more pigments and has a hydrophilic dispersing group bonded to the surface thereof.
Incidentally, the color reproduction of a blue region in a dark portion, particularly, a region where the L* value is 20 or less, has limitation. That is, since it is difficult to exhibit chromaticity and saturation when the luminance is low, a color having an L* value of 20 or less and an a* value of 30 or more or a b* value of −75 or less in the CIE/L*a*b* color system, for example, a color of (L*, a*, b*)=(19.4, 30.0, −72.6) or (L*, a*, b*)=(19.4, 37.6, −76.8), cannot be reproduced on an image recording medium with liquid droplets (ink). On the other hand, in an ink set, it is common to reproduce a color in a blue region by mixing a cyan ink composition and a magenta ink composition, a color in a red region by mixing a yellow ink composition and a magenta ink composition, and a color in a green region by mixing a yellow ink composition and a cyan ink composition. Therefore, when the color reproduction in a blue region is extended, it is necessary to simultaneously maintain the color reproduction ranges in other color regions, for example, considering the green region reproduced by a cyan ink composition and a yellow ink composition and the red region reproduced by a yellow ink composition and a magenta ink composition.
Accordingly, the present inventors have conducted intensive studies for extending the color reproduction range of a dark blue region and have proposed in a previous application (Japanese Patent Application No. 2008-215056) that an ink set composed of a combination of an cyan ink composition containing C.I. pigment blue 15:6 or C.I. pigment blue 60 as the pigment and a magenta ink composition containing a quinacridone-based pigment can extend the color reproduction range of a blue region in a dark portion with an L* value of 20 or less, in particular, that an ink set composed of a combination of an cyan ink composition containing C.I. pigment blue 15:6 as the pigment and a magenta ink composition containing a quinacridone solid solution pigment can produce a color having an L* value of 20 or less, an a* value of 30 or more, and a b* value of −75 or less. Such a color could not been reproduced with known ink sets.
However, this time, the inventors have found that C.I. pigment blue 60 contained in the cyan ink composition for extending the color reproduction range of a blue region in a dark portion largely reduces the chromogenicity of a green region.